don't know why, but i never even really considered keeping both eyes open, just thought it was normal to close one and use the peep with the other i guess. i'll try it later just for the heck of it to see what it's like though.
Heavy on the 'later' part. That's not something you want to do this close to hunting season. It can mess you up for a little while. Keeping both eyes open is the 'best' way to go. On the other hand, even though it works for most people, it does not work for all. You've got to do what works best for you, no matter what the experts decree is best.
A lot of being able to see where you hit depends on the fletching you use. If you use dark fletching, you'll never see it. That's why I've always liked bright fletching on my hunting arrows. If I'm worried about concealment, I use a camo cover over the fletches in my quiver.
Also, the faster an arrow is going, the harder it is to see. Once it gets over about 250 fps, my eyes have a hard time following it even in the best, brightest lighting conditions. It's hopeless in lower light.
Knowing where you hit is crucial to deciding how long to stay put and let the arrow do it's job before starting to trail the critter. Do whatever you need to do to see that arrow hit.